Some material and objects, when subjected to short wave electromagnetic radiation, will emit another type of electromagnetic radiation, usually with lower energy and longer wavelength. The general therm luminescence refers to the emission of electromagnetic radiation by a substance by absorption of incident radiation when only part of the absorbed energy is converted into heat (Ray, 1999). Since part of the energy is transformed into heat in the process, the emitted electromagnetic radiation is usually of longer wavelength. Luminescence exists in various types: if after the removal of the stimulus, the afterglow occurs within few nanoseconds and decays in less than 10 nanoseconds, the term fluorescence is used; if the afterglow persist for longer, up to many days, the term phosphorescence is used instead (Ultraviolet and Fluorescence Photography, 1974; Ray, 1999). Fluorescence photography, therefore, refers to capturing of light emitted by the object instantly, under the influence of shorter wave electromagnetic radiation.

In the most common case, the long ultraviolet wavelength is used to excite fluorescence in visible spectrum. Depending on the properties of the substances involved, the fluorescence can occur in any part of the visible spectrum, and in infrared. In case of this particular article, the ultraviolet-induced visible fluorescence photography refers to photographing visible light emitted by the objects under the influence of ultraviolet light (UV-A), and not to the photography of the UV light reflected from the object (Ultraviolet and Fluorescence Photography, 1974).

It is sometimes suggested to use blue light to induce fluorescence in the remaining part of the visible spectrum. It does appear to work better for underwater objects (Mazel, 2005). However, when dealing with plants, there are examples of flowers that emit bright blue light, often in addition to other colors, under the influence of UV, and excluding this blue light will give an incomplete picture of fluorescence.

Fluorescing scorpion Hadrurus arizonensis

Many natural objects and substances fluoresce under ultraviolet or blue light, including rocks and minerals (Warren et al., 1999), fungi and bacterial cultures (Williams & Williams, 1994), lichens and plants, hard corals and Anthozoa, crustaceans (Mazel et al., 2004) and spiders (Andrews et al., 2007), fish and birds (Arnold et al., 2002), body tissues and fluids, etc (Williams & Williams, 1994). Its function in nature is still insufficiently studied. For example, despite some statements in the popular literature (Bielmeier, 2009), the fluorescence in flowers is highly unlikely to play substantial role in biocommunication, such as a visual signal to attract pollinators, due to its much lower intensity, comparing to the normal flower reflectance of the visible light (Iriel & Lagorio, 2010).

Display of fluorescent minerals in the Swedish Museum of Natural History in Stockholm

Bielmeier (2009) also suggested that there is a correlation between the UV-dark areas (UV-absorbing areas) of flowers used to navigate pollinators, and their fluorescent properties: "...the plant would grow petals that are ultraviolet-reflective and a pistil or stamen that is UV-absorbent. To create dark areas, the plant produces pigments that absorb ultraviolet light - and as result fluoresce" (Bielmeier, 2009: 12). Even though this might be true for pistils and stamen of some plants, the statement does not really reflect the true nature of fluorescence in flowers as a whole. Fluorescence patterns of flowers do not necessarily correspond to the UV-reflectance/UV-absorbance patterns or to visible appearance of flowers.

There are flowers that are both UV-dark and are not fluorescent. There are also flowers that are fluorescing much brighter when they are already pollinated and their petals are drying out and dying. There are plants which fruits are fluorescing brighter than their flowers. The nature of fluorescence is much more complex than it may appear at a first glance. Moreover, this biological phenomenon is incredibly beautiful and interesting photographic subject.

Illumination

Photographing UV-induced visible fluorescence requires a powerful source of ultraviolet radiation, since fluorescence of many substances is often low and may otherwise require intolerably long exposures. With UV-fluorescence photography, it is important to ensure that only UV-light reaches the object, and that only the fluorescence is recorded by the camera. Fluorescence of natural objects is usually weak, compared to ambient light and needs to be performed in a completely dark environment. It makes such photography difficult to perform in nature, except for the night-time.

Sources of ultraviolet radiation must not produce any visible light, which can and will contaminate the output. Therefore, most of the UV-light sources need to be fitted with an ultraviolet-transmitting filter (excitation filter), which would block all other wavelengths of emitted light.

Electronic flashes with uncoated flash-bulbs will also produce sufficient amount of ultraviolet, in addition to visible and infrared light. They also need to be fitted with high quality UV-pass filter, in order to block all visible light. For photographers, the main drawback of the UV-induced visible fluorescence photography using flash is that the photographer does not see the fluorescence directly, making it more cumbersome to make exposure adjustments.

Currently available commercially UV LED torches offer a wide range of emitted wavelength, powers and prices. Many such UV LEDs, especially sold at lover prices, are often not accompanied with technical spec. sheets, and their power and output remains unknown.

Some such LEDs are not powerful enough to be used for fluorescence photography of natural objects, or have high output in visible spectrum (=contamination), or both. Therefore, it is highly recommended to purchase UV LEDs with known output specifications.

I personally use three UV LED torches with Nichia chips with the UV emission centered around 365nm. These types of torches were recommended to me by Dr. K. Schmitt. Such LEDs, albeit relatively expensive, have a powerful output (specified as 2-3 Watt for my torches but more powerful ones are already available) and very narrow emission spectrum, with very minimal output in visible (violet) range (Schmitt, 2010).

The protective glass of these torches was replaced with Schott UG1 glass in order to minimize even more the already minuscule leak of visible light by Nichia LEDs. Such torches are becoming cheaper and easily available from the online retailers.

In addition to the emitted fluorescence in a visible spectrum, depending on its properties, the subject may also reflect some of the ultraviolet light, which may contaminate or "overpower" the resulting image (see the picture of the sunflower above) and needs to be filtered from entering the camera by an additional ultraviolet absorbing filter (barrier filter). In general, the barrier filter should be able to block all radiation passed through the excitation filter (Ultraviolet and Fluorescence Photography, 1974).

White dead-nettle (Lamium album)

Rockcress (Phedimus spurius)

Light yellow filters such as Wratten 2A and 2B (or other filters with similar transmission curve) are often recommended for such purpose (Dorell, 1994), as they cut off all ultraviolet light and large part of the violet and blue.

Unfortunately, there are numerous objects in nature that emit fluoresce in the blue part of the spectrum, including many flowers. Therefore, the use of yellow filter will prevent the large part of fluorescence from being recorded in the picture. In such cases, UV-cut filters with "sharp" cut off curve can be used instead (Schmitt, 2009).

Moreover, special care should be taken when choosing a barrier filter, as some of the filters may fluoresce itself, contaminating the resulting image (Williams & Williams, 1994).

Myers (1981) and later Mazel (2005) experimented with photographing UV-induced visible fluorescence under the presence of ambient light, which for example makes underwater fluorescence photography much safer. If a fluorescence picture is taken in the presence of ambient light, the image will include the information from both fluorescence and from the ambient light reflected from the scene. Mazel (2005) estimated that for the resulting image to faithfully record the fluorescence, the image created by the reflected light should be considerably underexposed to the point that it is not recorded. He recommends the exposure of at least 3 f-stops less for the ambient light portion of the image, comparing to the image produced by fluorescence alone.

Both Myers (1981) and Mazel (2005) recommended the use of high power electronic flash in conjunction with the camera capable of fast flash-synchronization speeds (1/125th-1/500th of a second). Short exposure will thus drastically decrease the contribution of the ambient light, but the UV-induced visible fluorescence will be unaffected, as it only lasts as long as the duration of the flash.

Again, just like when using flash to induce fluorescence in the dark environment, using it in presence of ambient light has exact same drawbacks - the photographer can not see the fluorescence directly, and the exposure need to be set by trial and error.

Common daisy (Bellis perennis)

Striped squill (Puschkinia scilloides)

Just like in normal photography, using single light source, be it flash or LED, is very limiting. When using UV LED it is possible to create soft and uniform illumination by setting the exposure as long as technically feasible, in a range of at least 10-30 seconds, and than "painting" the object with UV light. When doing so, the exposure settings (aperture, shutter speed and ISO) will need to be determined by trial and error. "Painting" with light will not only allow creation of uniform (and often rather flat) illumination of a complex object with just one LED, but, with some experience, can be used creatively and can provide rather complex illumination, that, otherwise, would require multiple sources of light with adjustable output.

Post-processing

All the post-processing tools used for normal photography, can also be applied for UV-induced fluorescence photography, but there are two things worth mentioning in particular. As fluorescence tends to be a weak signal, capturing the image requires long exposures or use of high ISO settings, or even both, resulting in digital noise, than needs to be dealt with. What is more critical is that many live objects, especially flowers, may and will carry dust particles on their surface that can not be effectively cleaned up.

Many common types of fabric and paper are treated with fluorescent dyes. Washing powders also often contain "whiteners" that fluoresce very bright. Such fabric and paper dust is prevalent in urban environment and often fluoresces much brighter than the object itself, producing blown-out spots and patches in the resulting picture. These dust particles need to be manually removed or washed off - the method will depend on the physical properties of the object that is being photographed.

If such particles can not be physically removed before taking pictures of for example fragile flowers, they only way to deal with them in the post-processing is to use a "clone brush" or its equivalent, and remove them digitally.

Fluorescing lichens at 3X magnification

Several pictures in my gallery were taken at high magnification, 1:1-3:1, using either a high quality macro lens (such as Minolta 100mm F/2.8 Macro or Voigtländer APO Lanthar 125mm F/2.5) or a reversed enlarger lens (Apo-Rodagon 50mm F/2.8) fitted on bellows and focusing rack. In most such cases I use focus stacking technique to extend the depth of focus in my photographs: either a focusing rack or a macro lens with very long focus throw (APO-Lanthar) was used to create a series of images focused at certain intervals. Final images were produced using ZereneStacker software.

Safety

UV light is harmful. It is responsible for different types of skin cancer (Ultraviolet radiation and human health, 2009) and melanoma (Health effects of UV radiation, 2013). High intensity long-wave UV emitted by powerful LED sources falls into this category. Mercury vapor lamps, fluorescent tubes and xenon bulbs also produce hazardous amount of UV. Therefore, it is highly recommended to use appropriate eye and skin protection at all times when doing UV-induced visible fluorescence photography: high-quality UV-protective goggles and dark-colored non-fluorescent clothing.